Would the next step being applying this to the Field Cache?
That would be interesting! Or, maybe to DocValues instead...
Today neither DocValues nor FieldCache use an FST to hold the term data, but you're right we could explore this now, since we can lookup-by-ord or lookup-by-address (with this patch).
For example, the DocValues.BYTES_VAR/FIXED_SORTED_DEREF could hold the Term <-> ord/address map as an FST (doc -> ord/address would still be packed ints)... same for FieldCache.DocTermsIndex/.DocTermOrds This should be a sizable reduction in RAM required for the term data... but lookup time would get slower too.